Caenorhabditis elegans Embryo: Determination of Somatic Cell Fate

Abstract

Although embryogenesis appears to be determinate in C.
elegans because of its stereotype cell lineage, the mechanisms of
regional specification typical for indeterminate development are nevertheless
used. Early on, blastomere identities are specified by binary decisions, and
each blastomere gives rise to a coherent region of the embryo containing
different somatic cell fates.

Foundercell formation in Caenorhabditis elegans. (a) During the first fourdivision cleavages the fertilized zygote gives rise to the somatic foundercells in asymmetric divisions. The major tissues produced by the founder cellsare indicated. MS and C also produce some neurons. (b) Schematicrepresentation of the first divisions of the embryo. The first threeinductions specifying the AB lineage are indicated. Two later inductionsspecify minor left–right asymmetries in the AB lineage. These inductionsspecify blastomere identities corresponding to regions later in the larva (see Schnabeland Priess, ).

Figure 2.

Regionalorganization of the Caenorhabditis elegans embryo. (a) Twelve‐cellstage embryo. The lineage descent of cells is colour coded as shown at thebottom of the figure. (b) Dorsal, ventral left and right view of an embryo atthe premorphogenetic stage (380 cells). The descendants of the earlyblastomeres populate coherent regions. No mixing of the descendants ofblastomeres of the 12‐cell stage occurs. Cells form the regions by cellmigrations. (c) Three views of an embryo created in a computer simulation (M.Gumpel, unpublished). The blastomeres of the 12‐cell embryo shown in (a) weredivided only in the anterior–posterior direction. The descendantsintermingle and no coherent regions form. The specific arrangement of regionsin the C. elegans embryo therefore requires active sorting processes.(d) Arrangement of cells within a region at the premorphogenetic stage.Descendants of ABplpaa (red), ABplpap (yellow), ABplppa (green) and ABplpppp(blue) are shown. The anterior descendants of the next cleavage are shown witha lighter, the posterior ones with a darker tint. As indicated for the mostanterior lineages cells are not strictly arranged according to their lineagedescent within a region.

Figure 3.

The lineageproduces cell fates according to the positional requirements. The figure showsthe anterior part of a premorphogenetic embryo. Yellow nuclei (spheres),nervous system; blue, pharyngeal cells; green, intestine; grey,nonpharyngeal MS‐derived nuclei. The pharynx precursor is also outlined. TheMSaa derived cells (colour coded from red to orange) span through the anteriorof the embryo. The most anterior cell present at the premorphogenetic stagelies outside of the pharynx and later produces two neurons which contribute tothe part of the nervous system formed in this area from ABala. The two mostposterior cells are placed, like the most anterior, outside the pharynx andproduce body wall muscle. As indicated at the top of the figure the pharyngealcells acquire the fates needed in the specific positions to produce afunctional pharynx, for example at the margin of the intestine valve cells aremade which connect pharynx and intestine. The 28 cells derived from MSaapresent at this stage produce seven different tissues. The fates must bespecified primarily by positional criteria. The complex differentiationpattern indicates that no grouping according to cell fate occurs in thelineage.

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